This invention relates to a delay circuit in general, and more particularly to a smear-desmear filter capable of reducing adverse effects of the disturbances of instantaneous nature in data communication, such as short interruption.
The effectiveness of a smear-desmear filter was treated in a paper by Richard A. Wainwright published in IRE Transaction on Communication Systems, December, 1961, the contents of which will be briefly reviewed here.
When data signals with a frequency band width of O-f Hz are applied to a circuit with a delay characteristic of FIG. 1(a) (a smear circuit), a data signal of duration t is elongated to a duration of (t+T) and then deliverted to a transmission line. This elongated signal can be restored to the original data signal by passing it through another filter (desmear filter) with an inverse delay characteristic as shown in FIG. 1(b).
On the other hand, impulsive noises are generated on a transmission line, and hence, pass only through the desmear filter.
As a result, the energy of the noise power is dispersed over T seconds, and the effect of such noise power on data signals is reduced. In order to achieve any significant effects of such a smear-desmear filter, it is necessary to realize a steeply sloped linear function of delay on the frequency axis with a delay difference in a desired band width amounting to 50-100 times of the inverse of the band width. Due to difficulty in achieving such a delay difference, the smear-desmear filter has not been used on a commercial basis.
Impulsive noises generated by switches and the like, however, cannot be neglected, especially as the data density in data communication systems increases. Thus, there has been a pressing need for the development of a smear-desmear filter of superior characteristics.